Basement insulation -- XPS on just exterior walls?

Greetings all! Well the time has come for us to finish our basement "main room" and I have a quick question about the insulation of the walls.

First the layout: The room is essentially a square with 2 foundation walls (which obviously are against the earth/underground) and 2 interior walls (both made of concrete block -- separating us from other unfinished but heated basement rooms).

So, my insulation approach for the exterior walls is going to be to attach 1.5" Extruded Polystyrene Foam panels to the walls with the proper adhesive and then frame out the walls and if the 7.5R value of the XPS doesn't make the temp come up a little [it's about 65 degrees in that room in its current unfinished state -- and it's 9 degrees outside] do some fiberglass batting in the stud cavaties.

SO, my question is this!: This XPS isn't the cheapest stuff in the world and I am wondering if that is needed on the INterior walls? (the walls that separate this room we are finishing from the other basement rooms)? The walls of this room are like 27' long so that could save me a lot of money. And it will also save me the 1.5" of wall space tha I wouldn't have to come in without the XPS board. But of course I'm willing to buy the stuff if there's a compelling reason.

I have applied Polyurethane Foam closed-cell insulation materials. It’s materials offer high R values, one can use a thinner foam board to have the required thermal resistance one desires. This can be extremely helpful if there are space limitations involved. This strength involved the ability of the foam to resist deformation and the ability to main its shape when forced or loaded down.It can handles a wide temperature range, which makes it effective as a roof insulation. When combined with laminated aluminum foil facing, it is also an effective moisture and vapor barrier.

I would use polystyrene on the exterior walls with an r-13 to r-15 fiberglass insulation and then add a vapor barrier. You can just use the fiberglass on the interior walls with no vapor barrier or polystyrene. This would be the best r-value for your money.

That being said, spray foam will give you the best final product, however it is usually not a DIY product and much more costly.

I would use polystyrene on the exterior walls with an r-13 to r-15 fiberglass insulation and then add a vapor barrier. You can just use the fiberglass on the interior walls with no vapor barrier or polystyrene.

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Below grade walls can only dry to the inside of a structure. Using a vapor barrier as you suggest will trap moisture in the fiberglass insulation.

I would paint the exterior walls with a bast. waterproof paint, R-11 to R-15 with a vapor barrier is plenty of insulation. I would insulate the rim joists with R-19 and only about 2' in from the wall. I do not recommend the whole ceiling.